When information is recorded onto optical media, the laser power should be established at such a level as to provide well formed recording marks, whether the marks are represented by changes in the reflective polarization of a spot on a rewritable magneto-optical (MO) disk, by pits burned into the surface of an ablative write-once disk or by changes in reflectivity between amphorous and crystiline areas of a phase change (PC) disk. However, even if the laser power is fixed when an optical disk drive is manufactured, many factors can cause such a fixing to be less than optimum. For example, two pieces of media from different manufacturers, or even from different batches from the same manufacturer, may have slightly different characteristics and, therefore, react slightly differently to the application of the same laser power lever. Other factors which can affect the optimum laser power level include the age of the drive and media, the operating temperature of the drive, the temperature of the media and any temperature differential between the media and the drive the extent of media contamination, changes in the laser spot size and any focus or tracking sensor offsets.
As a result, techniques have been developed for drive self-calibration during use. In one such technique, a calibration pattern (which cannot be confused with user data) is recorded onto one or more sectors at varying laser power levels. The sector is read back and, based upon such parameters as readback signal amplitude, mark edge jitter, mark peak pulse position, mark-space asymmetry and mark length, the optimum laser power level is selected for recording operations. Calibration can be performed each time the drive is powered on, at predetermined intervals or each time a verification operation fails, among others. When a calibration is performed on an MO disk (or other re-writable media), a sector previously used for calibration can be erased and reused. Consequently, only a few such sectors need to be available. In contrast, however, when a calibration is performed on a write-once, read many (WORM) disk, a sector previously used for calibration cannot be reused. Many calibration sectors should be available when the disk is new. The requirement for many calibration sectors may be particularly acute for the recently proposed 130 mm 1.3 GB per side WORM media which uses a pulse width modulation (PWM) recording technique as compared to the currently common 130 mm 325 MB per side WORM media using a peak pulse modulation (PPM) recording technique. The four times increase in recording capacity is primarily due to the increase in recording density, decrease in mark size, and adoption of the PWM recording technique. PWM requires greater precision in mark-space writing because the transitions are used to encode the information. In addition, the PWM uses marks of varying lengths placing an additional demand on quality mark formation. All of these factors increase the need for use of optimum write power to form quality marks with precisely placed edges. In fact, in some operating environments, a drive may have to be calibrated each time a disk is mounted to achieve required data reliability and performance. It can be appreciated that if all of the calibration sectors are used, no further information can be recorded onto the disk because there is no longer the ability to ensure a proper laser write power level.
When a recorded data sector is determined during verification to be defective, the data must be written to another sector. In one disk format configuration for WORM media, one or more regions of the disk are set aside as spare sectors to replace defective "primary" sectors. Again, however, if the spare sectors are exhausted, no further data can be reliably recorded.